1. Field of the Invention
The present invention relates to a liquid immersion exposure technique, which projects a pattern of an original onto a substrate in a stage in which a gap between the projection optical system and the substrate is filled with a liquid.
2. Description of the Related Art
FIG. 3 shows a general structure of an exposure apparatus.
In FIG. 3, reference numeral 31 denotes a light source. In recent years, the light source 31 has shifted from an I-line to an excimer laser with a trend of shortening the wavelength due to miniaturization of exposure patterns, and even the laser source is shifted from KrF to ArF. Currently, in order to satisfy the demand of further miniaturization, the use of an F2 laser and EUV light are being investigated.
Light from the light source 31 passes through an introduction unit 32 and is led to an illumination optical system 33. At the illumination optical system 33, an illumination uniformity is eliminated and a beam is formed, and then, illumination light is irradiated onto a reticle 34, which is the original of the pattern to be exposed. The reticle 34, acting as an original, is placed on a reticle stage 35.
The light that has passed through the reticle 34 becomes the pattern light and is reduced-projected via a projection optical system 36, onto a wafer 37, as the substrate placed on an optical conjugated plane with the reticle 24.
The reticle 34 and the wafer 37 are each placed on a reticle stage 35 and a wafer stage 38 having a linear motor as a driving source, respectively, and exposure is repeatedly performed by step and repeat.
Further, a liquid immersion exposure apparatus, which has a gap between the projection optical system 36 and the wafer 37, filled with a liquid, such as pure water, has drawn attention in recent years. The liquid immersion method realizes a high-NA (numerical aperture) by the liquid having a high refractive index. This means that further miniaturization can easily be realized by providing an immersion liquid supplying unit to an existing ArF exposure apparatus without using an F2 and an EUV light source, which are burdensome to install.
FIG. 4 shows the structure of an immersion liquid supply system in a liquid immersion exposure apparatus.
In FIG. 4, a liquid immersion region is generated by a liquid immersion wall 21 on the bottom-most surface of the projection optical system 36, and a liquid supplying nozzle 22 and a liquid withdrawing nozzle 23 are placed in the liquid immersion region. Then, by supplying and recovering a predetermined amount of immersion liquid from the liquid supplying nozzle 22 and the liquid recovering nozzle 23, respectively, exposure is performed in a state where the liquid immersion region is filled with the immersion liquid.
The immersion liquid forms a part of the optical components, and thus, strict maintenance of purity, flow rate and temperature is required. Generally, ultra-pure water is used. The ultra-pure water produced in factory equipment is temperature-adjusted by a cooling device 24, a heater 25, a temperature sensor 26 and a temperature adjustment device 27, via a supply line 28, and is supplied to the liquid immersion region via the liquid supplying nozzle 22.
Further, a degassing unit 41 is provided with the supply line 28 and removes gases dissolved in the immersion liquid, thereby to attempt to reduce an exposure defect caused by micro-bubbles and to improve transmittance of the exposure light through the immersion liquid.
The above-mentioned improvement of transmittance not only improves productivity due to shortening of the exposure time, but, also, suppresses changes in the refractive index of the immersion liquid due to a rise in temperature caused by exposure energy. Thereby, good imaging performance can be stably obtained. See, for example, Japanese Patent Application Laid-Open no. 2004-282023 and Japanese Patent Application Laid-Open No. 2005-019615.
FIG. 5 shows the structure of a degassing unit.
In FIG. 5, tube-shaped hollow fiber membranes 42 are bundled together, and a degassing module 43, having a structure in which a space is separated into the membrane wall interior and the membrane wall exterior of the hollow fiber membrane 42, is arranged. An immersion liquid is supplied from a supply port 44 communicated with the membrane wall interior, and vacuuming is performed from an exterior supplying port 45, which is communicated with the membrane wall exterior. With this setup, degassing can be performed by discharging gases dissolved in the immersion liquid to the vacuum side of the membrane wall exterior via the hollow fiber membrane 42. The degassed immersion liquid is discharged from an interior discharge port 46 communicated with the membrane wall interior and returns back to the supply line 28.
However, the above-mentioned degassing unit utilizes a diffusion unit phenomenon of the membrane, which requires a vacuum pressure decrease, a membrane surface area increase, or making the membrane thinner, in order to improve the degassing efficiency.
On the other hand, there is a limit in the degassing capability, due to limitations in pressure resistance and space. When there is a change in the amount of dissolved gas prior to degassing, the change affects the degassed immersion liquid in the downstream side.
Further, the performance of the vacuum pump is closely related to the atmospheric pressure. Thus, a change of the atmospheric pressure may cause changes in degassing capabilities.
Further, oxygen, which is a component of the dissolved gases, is closely related to the transmittance of the immersion liquid, and a change of 0.73%/cm in transmittance occurs by a change in concentration of dissolved oxygen, by 1 ppm.
Thus, changes in the amount of dissolved gases in the liquid immersion exposure apparatus may occur when there is a change in the atmospheric pressure or in the amount of dissolved gases in the immersion liquid supplied from the factory equipment, which may cause an occurrence of micro-bubbles and deterioration of the optical performance.